1. Field of the Invention
This invention relates to the fields of olefin polymerization catalysts and processes for polymerizing olefins in the presence of chromium-containing supported catalysts.
2. Description of the Prior Art
The use of chromium compounds in the polymerization of olefins is well-known. U.S. Pat. Nos. 2,825,721 and 2,951,816 disclose the use of chromium trioxide supported on an inorganic material such as silica, alumina or combinations of silica and alumina, and activated by heating in reducing atmospheres, to polymerize olefins. When, however, a catalyst system of this type is used in techniques such as the well-known particle-form process, the resins produced, while useful in many applications, are unsatisfactory for others because of a deficiency in certain properties such as melt index.
Attempts to improve the properties of polyolefins produced using supported, heat-activated chromium oxide catalysts have been made by adding various compounds to the supported chromium oxide prior to the heat activation thereof. For example, U.S. Pat. No. 3,622,522 shows that an alkoxide of gallium or tin may be added to supported chromium oxide prior to heat activation (and compares unfavorably the addition of aluminum isopropoxide). U.S. Pat. No. 3,715,321 suggests adding a compound of a Group II-A or Group III-B metal to supported chromium oxide prior to heat treatment, whereas U.S. Pat. No. 3,780,011 discloses adding alkyl esters of titanium, vanadium or boron, and U.S. Pat. No. 3,484,428 discloses adding alkyl boranes to such catalyst.
It is also known to utilize other chromium compounds in the preparation of olefin polymerization catalysts. Such compounds include various silyl chromate and polyalicyclic chromate esters as described, for example, in U.S. Pat. Nos. 3,324,095; 3,324,101; 3,642,749; and 3,704,287. Other such catalysts are disclosed in U.S. Pat. Nos. 3,474,080 and 3,985,676, and commonly assigned application Ser. No. 759,213, filed Jan. 13, 1977, now U.S. Pat. No. 4,100,104. These catalysts may be employed with organometallic reducing agents such as, for example, trialkyl aluminum compounds.
It has been disclosed in U.S. Pat. Nos. 3,984,351 and 4,049,896 that the properties of olefin polymers, e.g., melt indexes, can be substantially improved by the use of a catalyst prepared by depositing an organophosphoryl chromium compound and an aluminum compound such as aluminum sec-butoxide on an inorganic support material and heat-activating the supported composition in a nonreducing, preferably oxygen-containing atmosphere, at a temperature of from about 300.degree. C. up to the decomposition temperature of the support. The resulting material which is preferably combined with a metallic and/or non-metallic reducing agent, provides a catalyst system capable of producing polymers having improved flow properties and shear responses in addition to increased melt indexes.
It is among the objects of the present invention to provide another improved technique for preparing olefin polymerization catalysts, and a polymerization process utilizing the same which results in the formation of polymers having high melt indexes and other desired characteristics, such as variations in the molecular weight distribution.